What's wrong is that you asked for any usecase and then critique one because it's not broad enough for you.

Sorry, I'm not trying to be argumentative, I just argue in order to understand better.

Then argue honestly and work to steelman other's arguments. To address your point, I'd hardly consider IoT platforms to be a "narrow" usecase. Smaller than the whole of computing, surely, but it's a growing field. The reality is that more and more devices will become available that generate all sorts of hard to predict data. Being able to handle those easily will be a large strength for platforms going forward. Dropping this hard to predict data as JSON into a RDMS will certainly come back to haunt you in 5 years.

How will it come back to haunt? Again, just curious.

You'll have dumped it into a strict database, giving yourself a false sense of order and organization. But later when you need to query that amorphous data, you might be able to use OPENJSON or something else, but a NoSQL solution will have been built to handle this type of query specifically with utilities like key exists and better handling for keys missing or only sometimes being present.